Field of the Invention
The present invention relates to an inspection apparatus and an inspection method. Specifically, for example, the present invention relates to an inspection apparatus that inspects a pattern defect by acquiring an optical image of the pattern image on the substrate by applying laser irradiation thereto.
Description of Related Art
In recent years, with the advance of high integration and large capacity of a large scale integrated circuit (LSI), the line width (critical dimension) required for circuits of semiconductor elements is becoming progressively narrower. Such semiconductor elements are manufactured by exposing and transferring a pattern onto a wafer to form a circuit by means of a reduced projection exposure apparatus known as a stepper while using an original or “master” pattern (also called a mask or a reticle, and hereinafter generically referred to as a mask) with a circuit pattern formed thereon. Then, in fabricating a mask used for transferring (printing) such a fine circuit pattern onto a wafer, a pattern writing apparatus capable of writing or “drawing” fine circuit patterns by using electron beams needs to be employed. Pattern circuits may be written directly on the wafer by the pattern writing apparatus.
Since LSI manufacturing requires a tremendous amount of manufacturing cost, it is crucial to improve its yield. However, as typified by a 10-gigabit DRAM (Dynamic Random Access Memory), the scale of a pattern configuring an LSI has changed from on the order of submicrons to nanometers. One of major factors that decrease the yield of the LSI manufacturing is a pattern defect of a mask used when, by the photolithography technology, exposing and transferring (printing) a fine pattern onto a semiconductor wafer. In recent years, with miniaturization of dimensions of an LSI pattern formed on a semiconductor wafer, dimensions to be detected as a pattern defect have become extremely small. Thus, the pattern inspection apparatus that inspects a defect of a transfer mask used in manufacturing LSI needs to be highly precise.
As an inspection method, there is known a method of comparing an optical image of a pattern formed on a target object or “sample”, such as a lithography mask, imaged at a predetermined magnification by using a magnification optical system, with design data or an optical image obtained by imaging the same pattern on the target object. For example, the following is known as pattern inspection methods: the “die-to-die inspection” method that compares data of optical images of identical patterns at different positions on the same mask; and the “die-to-database inspection” method that inputs, into the inspection apparatus, writing data (design pattern data) generated by converting pattern-designed CAD data into a writing apparatus specific format to be input to the writing apparatus when a pattern is written on the mask, generates design image data (reference image) based on the input writing data, and compares the generated design image data with an optical image (serving as measurement data) obtained by imaging the pattern. According to the inspection method for use in the inspection apparatus, a target object is placed on the stage so that a light flux may scan the object by the movement of the stage in order to perform an inspection. Specifically, the target object is irradiated with a light flux from the light source by the illumination optical system. Light transmitted through the target object or reflected therefrom is focused on a sensor through the optical system. An image captured by the sensor is transmitted as measurement data to the comparison circuit. In the comparison circuit, after performing position adjustment of images, measurement data and reference data are compared with each other in accordance with an appropriate algorithm. If there is no matching between the compared data, it is determined that there exists a pattern defect.
In mask inspection, in addition to inspecting a pattern defect (shape defect), it is also required to measure a pattern positional deviation. Conventionally, measuring a pattern positional deviation has been performed using a dedicated measuring device. If the apparatus that performs pattern defect inspection can measure a pattern positional deviation simultaneously with the inspection of a pattern defect, a significant advantage cost-wise and inspection-time-wise can be achieved. Therefore, the inspection apparatus is increasingly requested to have such a measurement function.
As to the pattern inspection apparatus, there is a case where the pattern forming surface where patterns are formed is located in the downward direction. This is for preventing a falling foreign substance from adhering to the pattern forming surface, for example. Moreover, this is because, in the exposure apparatus that transfers (prints) a pattern with using a mask, the pattern forming surface of the mask is located in the downward direction in many cases. In the pattern inspection apparatus, it is required to mount therein a light transmission inspection optical system that emits a laser light on the pattern forming surface in order to perform detection using a transmitted light, and therefore, it is difficult to support the whole mask surface to be appressed to a stage. Accordingly, the stage supports the external side of the pattern forming surface. With such a configuration, the mask is deflected (bent) by its own weight. Therefore, the pattern forming surface is bent outward by the deflection, which results in positional deviation of the pattern forming position.
By contrast, as to the writing apparatus that writes a pattern on the mask, the mask substrate is arranged such that the pattern forming surface is in the upward direction to write a pattern thereon. Also, in the writing apparatus, the whole back side of the mask is not supported either. Therefore, the mask is deflected by its own weight. However, in the writing apparatus, the writing position is corrected considering such deflection in advance. In other words, writing is performed such that a pattern is formed at a desired position when the mask is ideally horizontally arranged.
When inspecting the mask on which a pattern has been written as described above by a pattern inspection apparatus, deflection (bending) occurs also in the inspection apparatus. Therefore, even if the pattern forming position is corrected in the writing apparatus, positional deviation arises at the time of inspection by the inspection apparatus. In shape defect inspection, since determination is performed after a position adjustment between an image of a measured pattern and an image of a design pattern, even if positional deviation occurs because of such deflection, the deviation has been conventionally permitted. However, in measuring positional deviation of a pattern, positional deviation due to deflection is an unacceptable problem because it is concerned with positional deviation itself.
With regard to measurement of a CD (critical dimension) deviation, though it is not about positional deviation of a pattern, there is proposed an inspection method in which a pattern line width (critical dimension) in an image obtained for each preset region is measured, a difference from design data is calculated, and an average of all the CD differences in each region is compared with a threshold value, so that an irregular line width region is found as a CD error (dimension defect) (refer to, e.g., Japanese Patent No. 3824542).